The high frequency (HF) portion of the electromagnetic spectrum includes radio frequencies in the range of approximately 2 to 30 MHz. The HF band is suitable for transmission of ground wave radio signals for short distances (e.g., up to about 40 km), as well as skywave radio signals for longer distances. Near Vertical Incidence Skywave (NVIS) involves the propagation of radio waves, which are refracted by the ionosphere and return to the ground at a certain radius with respect to the point of origin. The NVIS propagation is implemented at acute elevation angles (e.g., 70°-90° relative to the horizontal), providing omni-directional transmission for distances up to about 300 km. Ionospheric refraction will fail to occur beyond certain frequencies, and so NVIS propagation operates best at the lower end of the HF band (e.g., approximately 2-12 MHz). These frequencies however are particularly susceptible to atmospheric noise. The ionosphere is an atmospheric layer that is ionized by solar radiation, and the refractive characteristics of the ionosphere changes based on the time of day, season of the year, location of the sun, and various additional factors which are constantly varying.
Assurance of reliable HF radio communication necessitates the employment of suitable antennas. A whip antenna is a fairly prevalent monopole antenna composed of a single straight vertical conductor element that functions as a radiating element and is mounted atop a conducting surface (ground plane). Whip antennas are commonly used aboard a vehicle or on a portable handheld transceiver, as they do not require extensive assembly or deployment operations for establishing a communication link while the vehicle or portable transceiver is in motion. Whip antennas are particularly suitable for providing ground wave radio propagation, which requires a vertically polarized antenna configuration. However, for NVIS propagation, a whip antenna provides very low gain when radiating at high elevation angles. There is a practice of utilizing a dipole antenna aboard vehicles or portable transceivers to enable NVIS communication. However, the deployment of a dipole antenna typically requires installing at least one mast for supporting the dipole wires. The installation of masts is time consuming, and increases the overall weight and cost of the antenna.
A narrow-band dipole antenna that is adapted for use at a particular frequency features relatively high gain at high elevation angles, but the dipole must be at least a certain length. For example, such a dipole antenna operating at 2 MHz has a length of approximately 74 m, which is impractical for use in many applications. Furthermore, the efficiency of the Automatic Link Establishment (ALE) systems is drastically reduced by limiting operation to a very narrow frequency band and obligating the frequent changing of the dipole length. Efficient wide-band dipole antennas generally require a large installation field (e.g., 50 m in length and 15 m in width), resulting in considerable weight due to the antenna mast and all the other necessary accessories.
U.S. Pat. No. 4,217,591 to Czerwinski et al, entitled “High frequency roll-bar loop antenna”, is directed to a vehicular mounted vertical loop HF communication antenna. The antenna includes a metallic base horizontally positioned on the vehicle, and a vertically positioned metallic loop element with one end affixed to the metallic base. The antenna further includes a variable capacitor connected between the other end of the metallic loop element and the metallic base, to form a closed transmitting loop. The antenna further includes a coupling loop (e.g., of coaxial cable) coplanar with the metallic loop element and slidably mounted on the metallic base, where the area under the loop is adjustable to maintain a desired input impedance at the antenna input terminal. The antenna is adapted to provide NVIS operation over one HF frequency range, and vertically polarized whip antenna operation over another higher HF frequency range.
U.S. Pat. No. 4,433,336 to Carr, entitled “Three-element antenna formed of orthogonal loops mounted on a monopole”, is directed to an antenna which is electrically steerable in a transmitting mode and which is capable of distinguishing the direction from which signals are received without a mechanical rotator. The antenna includes two loop antennas mounted on top of, and electrically coupled to, a vertically oriented monopole antenna, where the respective axes are orthogonal to one another. Each loop antenna includes an outer primary loop and a smaller inner secondary loop disposed in the same plane as the primary loop. The primary loops and secondary loops are interrupted at their top ends opposite where they join the monopole antenna. A tuning capacitor is coupled between the halves of the two primary loops, and coaxial cables feed the two secondary loops. The antenna may selectively radiate either omnidirectionally (via the monopole antenna) or directionally (via the loop antennas).
U.S. Pat. No. 5,252,985 to Christinsin, entitled “Whip tilt adapter”, is directed to an adapter that enables adjusting the polarization of an HF radio whip antenna. The adapter includes a near-horizontal member pivotally attached to a vertical shaft. The near-horizontal member is held stationary with respect to the vertical shaft by securing means, such as a pin that is inserted through matching sets of holes through the member and shaft. The whip antenna is inserted into a horizontal port at the distal end of the near-horizontal member. When the member is oriented substantially horizontally, the antenna provides NVIS operation utilizing reflective/refractive characteristics of the ionosphere (e.g., at 2-14 MHz). When the near-horizontal member is oriented vertically, the antenna provides for short-distance HF groundwave communication. The member may also be oriented at various angles in between a horizontal and vertical orientation. The adapter may be installed on a mounting base on a vehicle.
U.S. Pat. No. 6,917,339 to Li et al, entitled “Multi-band broadband planar antennas”, is directed to planar antenna with multi-band and broadband functionalities applicable for compact antenna applications (e.g., at around 2-5 GHz). The antenna includes two inverted-L antennas (ILAs) facing each other across a gap. One of the ILAs is input fed (e.g., directly by a coaxial cable input), and the other ILA is electromagnetically coupled to the fed ILA. The vertical legs of the two ILAs are parallel and substantially the same length, while the horizontal leg of the fed ILA is shorter than the horizontal leg of the coupled ILA. The position of the gap affects the bandwidth of the antenna. In one embodiment, a dual-band antenna includes a monopole antenna disposed between the ILAs. The monopole receives input fed and is connected to the input fed ILA near its base. The monopole is designed for resonance at a higher frequency than the ILAs.
U.S. Pat. No. 7,839,344 to Marrocco et al, entitled “Wideband multifunction antenna operating in the HF range, particularly for naval installations”, is directed to a wideband linear HF antenna designed particularly for fixed installations onboard naval units for military communications. The antenna includes a plurality of radiating elements forming conducting branches arranged in a bifolded configuration (i.e., two closed nested coplanar paths). The antenna further includes electrical impedance elements disposed within the conducting branches, to selectively impede current flow within selected frequency ranges to establish current paths according to the operating frequency. The antenna is adapted to provide uniform radiation at different angles of elevation for the entire HF band. In particular: NVIS communication at the lower HF range (2-4 MHz) and shorter distances (up to 150 km); sea wave and ionospheric reflection communication at low HF frequencies (2-7 MHz) and slightly greater distances (up to 500 km); ionospheric reflection communication at medium HF frequencies (6-15 MHz) and medium distances (1000-2000 km); and communication at low-medium angles of elevation (5-30° at higher HF frequencies (15-20 MHz).
Additional dual antenna arrangements which combine monopoles and dipoles can be found in Japanese Patent Publication No. 5041610(A) to Taniyoshi, entitled: “Antenna for mobile body”; Japanese Patent Publication No. 52101949(A) to Kawai et al, entitled: “Antenna apparatus”; and Japanese Patent Publication No. 2002-100928(A) to Inoue, entitled: “Composite antenna”.